The main benefit/impact is the higher frequencies that pass through the port have to be reflected before they reach the listener. This is purely my opinion, based only on theory, but a speaker should not have it's port on the front baffle if the driver has any significant output at the first port resonance.

I have heard that if you have a reasrward facing port, the sound waves stay in phase. then again this suggests that a port on the same plain as your speaker creates out of phase waves so i dont know how true that theory is. i have tried front, rear and sise firing ports, they're pretty much the same if you are using normal- what's more important is the size of the port - and of course the size of the box. if the port is too big you lose compression and the driver distorts easier. you just gona have to mess around.

The wavelength of sound at the frequencies a port works is so long that the position of the port is irrelevant. Front, back, top, bottom, put it where you will.

__________________
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?

It said that best result for in-room response is to use several subs at different locations. In line with above logic, how about subwofer with two ports? One port phacing front - the other port at side or rear. I never seen that. Usually subs with two ports have the ports close and in same diection.

Same thing, wavelengths of the frequencies of interest are so long that splitting them around the speaker will make no difference at all. Distributed subs work because they are spaced metres apart, and that is a significant proportion of the wavelength.

__________________
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?

Only one to my knowledge and experience: reducing the amount of midrange escaping from it. Even some manufacturers use rear facing port AND woofer:http://www.verityaudio.com/ :
<<<﻿Our solution is to place the woofer rearward and use a first-order filter. We then benefit from the
room bass reinforcement and benefit from a natural
acoustical filtration of the mid-frequencies that are left over by the electrical filter.<<<

Obviously, its real impact will partly depend of the walls reflection of the listening room.

Whenever possible, I always locate the port on the bottom of the cabinet and include an angled floor-board that allows the pressure wave out on three sides(F+L+R). This floor board also takes care of carpeting issues. I find this gives the greatest floor gain, gives the most uniform room gain, removes any direct path between higher frequencies off the rear-cone and the listener, and often allows use of the longest straight port tube since most cabinets are higher than deep. For a 3-way, this allows putting the woofer high off the floor and close to the midrange while still getting full floor gain.

I favor stereo subs located near/under the front speakers. I can hear bass speaker positioning at 80Hz crossover. Two woofers also reduces room effects.